2D electron-hole "superconductor": Topological excitonic insulator

Gate dependence of the conductance for a macroscopic Corbino device under inplane magnetic field from 0 T to 35T.Lingjie Du, Rice University

Decades ago a mechanism was proposed that described a quantum phase transition to an insulating ground state from a semi-metal (excitonic insulator, or EI) using very similar mechanics to those found in the BCS description of superconductivity. The discovery of this transition to an EI in InAs/GaSb quantum wells is striking not only for the long-sought experimental realization of important physics, but also the presence of recently proposed topological behavior.

What did scientists discover?

We observe a topological excitonic insulator in double-quantum-well semiconductor devices. By applying electric fields to the device, MagLab users tuned the electron and hole densities to such low values that electrons in one quantum well pair up with holes in the other layer. They measured the resulting excitonic insulator energy gap and quantized edge states. The energy gap and edge states exist in an in-plane magnetic field up to 35 teslas, where interlayer tunneling is eliminated. Persistence of the insulating state into this high-magnetic-field regime confirms that the energy gap originates from the exciton insulator state.

Why is this important?

The physics that produces an excitonic insulator is a form of pairing of quasiparticles, similar to that which produces a superconductor. These results address a question posed in physics some fifty years ago and verify a recent prediction regarding the potential topological properties of this double quantum well system.

Who did the research?

Why did they need the MagLab?

To confirm the excitonic insulator gap and its topological origin, researchers needed to apply a large (35 T) magnetic field in the plane of the quantum wells to adequately separate electron and hole energy bands in momentum space. The study also relied on the ultra-low temperatures at the MagLab, instrumentation that partners with the MagLab’s ultra-high magnetic fields to enable this research achievement.